Electrolytic process for the manufacture of litharge



R. G. BowMAN 5r Al.

Filed oct. 3o, 19:55

ELECTOLYTIC PROCESS FOR THE MANUFACTURE 0F LITHARGE May 16, 1939.

Patented May 16, 19.39

ELEC'IROLYTIQ PRCESS EOE THE MANU? FACTURE F LITHARGE UNITED STATES PATENT ori-fics l Reginald G. Bowman, Gary,

Knox, Jr., Hammond, Ind., conda Lead Products Company,

'and William J. assignors to Ana- New York,l

N. Y., a corporation of Delaware Application October 30, 1935, Serial No. 47,340

13 Claims.

lead. The product of these methods does not l meet the high standard of purity and uniformity demanded by consumers of litharge, and; as a consequence, manufacturers have had to resort to careful' selection and grading of the product. rejecting a. considerable portion as unsuited for gn many of the uses for which high quality litharge is required.

It has been proposed to overcome the disadvantages inherent inthe production of litharge by direct oxidation of molten lead by producing litharge electrolytically, but, heretofore, no commercially feasible electrolytic process for the production of litharge has been devised. Probably this has been owing to a lack of understanding of factors influencing the electrolytic reac- :in tions occurring during operation of the various processes proposed. Whatever the reason may have been, however, we have found that lt is possible so to control the progress of the electrolytic reactions resulting in the production of :is litharge as to make possible the development of a satisfactory cnmercial electrolytic process.

Electrolytic litharge is generally produced in a bifluid electrolytic cell, that is, a cell in which Y two physically separated electrolytes are emi 4o ployed. One of these electrolytes, the anolyte,

is in contact with the anode, and the other, the

catholyte, is in contact with the cathode. .Physical separation of the anolyte and catholyte is effected by, means of a permeable or semi-perme 4s able membrane or diaphragm of such nature and so arranged as to permit the passage of electric current from anode to cathode through the anolyte and the catholyte.

The anode comprises a sheet or plate of me;

so tallic lead, and the anolyte with-winch it is-in contact comprises an aqueous solution ci a salt,-

for example, an alkali metal salt such as sodium acetate, capable upon electrolysis of providing a solvent. furthe lead of the anode. The catholyte 55 comprlsanaqueous solution ot a substance, for

example. sodium hydroxide, capable of acting as a reagent for precipitating the dissolved lead in the anolyte as litharge. The cathode in contact with the catholyte is preferably insoluble in the catholyte and may comprise a thin sheet of copper, iron, or other suitable conducting material.

The diaphragm, which constitutes an important element in biiluid cell construction and operation, advantageously comprises a bag or envelope surrounding one of the electrodes, preferably the cathode. Fabrics such as linen woven to attain an optimum degree of porosity serve admirably for use in thel construction '-of the diaphragm.

The reactions taking place during electrolysis apparently proceed somewhat as follows: When an electric current is passed through a-cell of the type described, employing a sodium acetate solution as the anolyte and a solution of sodium hydroxide as the catholyte, lead from the anode dissolves as normal lead acetate, and sodium ions are caused, by the action of the current, to migrate toward the cathode. They pass through the diaphragm and enter the catholyte, wherein, at the cathode, they react with the water present to form sodium hydroxide, liberating hydrogen. The sodium hydroxide thus formed. in the catholyte diiuses back through the diaphragm into the anolyt and there it reacts withthe normal lead acetate to form a soluble basic acetate of lead and to regenerate a portion of the sodium acetate. The basic acetate of lelad'reacts with a further quantity of. sodium hydroxide to produce yellow crystalline flakes of litharge, which precipitate from solution, and simultaneously the balance of the sodium acetate is regenerated.

We have found that the above-outlined process will not .proceed satisfactorily unless the con-v centration of dissolved lead in the anolyte is maintained within predetermined limits, and, moreover, satisfactory operation requires that the lead in solution be present as the basic lead ion and not as the plumbite ion. If the anolyte contains an insuillcient quantity of basic lead acetate in solution, the rate at which sodium hydroxide migrates from anolyte is apt to be such that all ofthe lead willhe precipitated. leaving the anolyte alkaline. The lead -dissolved at the anode will then be converted to plumbite salts, which are incapable of being precipitated as litharge, and the process will become inoperative. Conversely, if the concentration ci basic lead acetate exceeds the the catholyte into the maximum allowable limit, local variations in by maintaining the concentration of temperature and concentration Will cause crystallization of the basic lead acetate along with precipitation of litharge, resulting in a contaminated product. `Since lead forms a series of basic lead acetatesof different solubilities exerting different effects upon the pH of their solutions,.it is difficult to define exactly the limits within which the concentration of lead should be maintained. 'I'he practical limits between which the concentration of dissolved lead, calculated as metallic lead in the anolyte, shold lie, however, have been found to be about 0.1% to 1.0%. Especially satisfactory results have been attained the dissolved lead at between 0.2% and 0.4%, say about 0.3%, of the weight of the solution.

In accordance with the present invention, the concentration of dissolved lead -in the anolyte is controlled by regulating the rate at which the sodium hydroxide or other litharge precipitant is transferred from the catholyte to the anolyte. 'I'his may be accomplished by selecting as the diaphragm a material of suitable porosity, by regulating the extent by which the hydrostatic head of liquid in the catholyte compartment exceeds that of the liquid in the anolyte compartment, by adding catholyte to anolyte at any point outside the reacting zone of the cell, or by employing any -suitable combination of these methods of control. Preferably the control afforded by the diaphragm porosity or the hydrostatic head of anolyte with respect to catholyte,

or both, should be such that the rate of transfer of hydroxyl ions as effected thereby approximates but does not exceed the rate at which the lead dissolved in the anolyte is capable of combining with said hydroxyl ions to produce litharge.

Complete control is then attained by suitable' addition of catholyte to anolyte.

A specic embodiment of the invention is diagrammatically illustrated in the accompanying fiowsheet. An eleotrolytic cell |10 is schematically shown as having a diaphragm II to separate an anolyte compartment I2 from a catholyte compartment I3. A lead anode (not shown) is in contact with anolyte within the anolyte compartment I2, and a copper, iron or other suitable insoluble cathode (not shown) is in contact with catholyte within the catholyte compartment I3. (In a commercial cell, of course, there would be a large number of alternate anolyte and catholyte compartments, each compartment containing an anode or a cathode.) The anolyte within the anolyte compartment comprises an aqueous solution containing about ten percent sodium acetate, and the catholyte within the catholyte compartment comprises an aqueousY solution containing about ve percent sodium hydroxide.

'In order to maintain proper quantities and concentrations of electrolyte within the cell, and to provide for easy recovery of the products of the cell, both anolyte and catholyte are circulated through systems external to the cell itself. A reserve of anolyte is maintainedin an anolyte storage tank I4, from which quantities are delivered as required to an anolyte feed tank I5. From the feed tank, anolyte is supplied at a substantially constant rate to the anolyte compartment I2 of the cell. Excess anolyte is permitted to overflow therefrom and run to a settling tank I6, ,and a further quantity of anolyte, withdrawn either continuously or intermittently from a low point in the anolyte compartment together with the precipitated litharge or other product of the cell, is likewise delivered to the settling tank I6.

Quantities of anolyte may also be passed from the storage tank I4, feed tank I5, or both, directly to the settling tank I6, for reasons explained in detail hereinafter.

In the settling tank I6 the bulk of the litharge or other solid product of the cell settles from the liquid and is withdrawn from a point at or near the bottom of the tank to a filter II. The supernatant liquor from the settling tank I6, together with such solid material as has notsettled, passes to a. second settling tank I8. Here the balance of the solid matter settles from the solution and is withdrawn from a point adjacent the bottom of the second settling tank I8 and passed to the filter I'I. y

'Ihe bulk of the liquor still. associatedwith the sold litharge or other product is removed in the filter I1, and the filtered residue is washed with water, preferably countercurrently while still on the filter. The solid product may then be passed to a drying oven 20 to remove the last traces of water, and after drying may be ground if desired v in a ball mill 2| or other appropriate grinding apparatus, from which is obtained the final product.

tling tank I8'and the anolyte recovered in the the soluble salts of the anolyte, also may advantageously be united with the anolyte in the anolyte sump 22. 'Anolyte from the sump may be delivered by means of a pump 23 or otherwise to the storage tank I4 for recirculation through the system.

ACatholyte is circulated similarly to the anolyte through a system external to the cell. Catholyte from a catholyte storage tank 24 passes to a catholyte feed tank 25 and from there to the catholyte compartment I3 of the cell. Excess catholyte overowsfrom the catholyte compartment and is delivered to a catholyte sump 26, from which it may be pumped by means of a pump 21 to the catholyte storage tank 24for recirculation through the system.

A quantity of catholyte is withdrawn from the catholyte feed tank 25 (or from the catholyte storage, tank 24 if desired) either continuously or intermittently as may be required and is added to the anolyte withdrawn from the lower portion of the anolyte compartment I2 of the cell. Make-up water is supplied from time to time as needed to the catholyte storage tank 24.

To start the cell. in operation, it is necessary to supply dissolved lead in the form of lead acetate vto the anolyte. This may be done` by simply passing current through the cell to dissolve lead from the anode in the form of lead acetate, whileretarding precipitation thereof as litharge until the desired concentration of dissolved lead in the anolyte has been attained.'

With large volumes of anolyte being circulated, however, considerable time would lbe required before this could be accomplished, and consequently, it is preferred to make up the anolyte with the proper quantity of lead acetate already dissolved therein. In preparing the anolyte, it has been found best to make up a saturated solution of sodium acetate and to add sufficient lead acetate so that when the solution is diluted to the proper strength for use as the anolyte, the solution will contain about 0.3% of dissolved lead and about 10% sodium acetate, both by weight of the solution. 'Ihis procedure offers the additional advantages of effecting precipitation as The supernatant liquor from the second setides,. and these precipitated impurities maybe separated from the solution before it is introduced into the anolyte circuit.

It is also preferred, especially if the anolyte is prepared as described above, to prepare a concentrated sodium hydroxide solution and to dilute it to the proper strength (about 5% NaOH by weight of the solution) for use as the catholyte. Practically, this procedure is more advantageous than depending upon electrolysis for the production of a suitable catholyte. Sodium carbonate, which is apt to be the principal impurity in the sodium hydroxide employed, may be separated from the concentrated solution prior to dilution for use as the catholyte, in any suitable manner, as by fractional crystallization. After the process has been set in operation, no further addition of reagents (sodium hydroxide and sodium acetate) is necessary.

After the anolyte and catholyte compartments have been filled with anolyte and catholyte, re-

by passing'an spectively, electrolysis is started electric current through the cell. Lead dissolves from the anode and litharge commencesto form in the anolyte, probably substantially in accordance with the reactions mentioned. above. The

litharge settles in the anolyte to the lower po'rtion of the cell, from which it is withdrawn together with a quantity of anolyte and is delivered to the first settling tank I6 for purposes of z recovery substantially as described above. The

recirculating anolyte and catholyte may be used over and over, since the sodium acetate is regenerated and lead acetate and sodium hydroxide are produced by electrolysis in the same chemical proportions as they are consumed in the production' of litharge. Thus the only reagents actually consumed are the lead of the anode, which may be replaced by introducing a new anode from time to time as necessary, and water which is decomposed to produce the sodium hydroxide employed in the precipitation of the litharge; water\added to the catholyte in the catholyte storage tank or the catholyte feed tank serves to replace the water thus consumed.

As pointed out above, continued successful operation of the-cell is dependent upon maintaining the concentration of dissolved lead within proper limits, advantageously about 0.3%. This is accomplished by regulation of the rate at which dissolved lead is removed as litharge from the anolyte with respect to the rate at which it is added to the anolyte byvelectrolysis. In practice, this means controlling the rate at-which hydroxyl ions of Athe catholyte are transferred to the anolyte. VThe diaphragm affords animportant element of control for this purpose, and it should be selected so as to have about that deu gree of porosity which will permit migration of sodium hydroxide through itat a rate approximating but not exceeding the rate at which it is required in the anolyte acetate dissolved at the anode. If the diaphragm is suiciently porous to permit migration of the sodium hydroxide at just the rate at which it is consumed in the anolyte, the entire control pf the process hinges upon the diaphragm, and a l slight failure thereof may throw the entire process out of balance. `If the degree of porosity of the diaphragm Aexceeds this critical limit, easy control of the, process will not be possible. Conseto react with Vthe lead spect to the catholyte, a considerable measure of control over the'rate of seepage of catholyte to anolyte, and thus of the transfer of hydroxyl ions from catholyte to anolyte, is provided. In general, a hydrostatic head of a few inches in favor of the catholyte will be satisfactory.

In practice it is rather diiiicult to vary the hydrostatic head of anolyte with respect to catholyte to any very great degree without providing a cell of relatively expensive construction,v and furthermore, variations in the hydrostatic head of the catholyte are apt to beA sluggish in bringing about the effect they are intended to produce. It is, therefore, preferred to operate the cell in lsuch a manner that, even when the catholyte benefits from a slight hydrostatic head with respect tofthe anolyte, the rate at which sodium hydroxide migrates'through the diaphragm into the catholyte is less than the rate at which it is consumed. Complete balance of the process is then achieved .by adding quantities of catholyte containing sodium hydroxide to the anolyte at some point external to the reacting zone of the cell. For this purpose, it is preferred to add catholyte from the catholyte feed tank 25 to the does not pass through the cell directly to the'y first settling tank I6, in order that the dissolved lead content thereof may be controlled, and to prevent the accumulation of quantities of anolyte of varying dissolved lead content in the various vessels employed. Thismethod .of regulating the dissolved lead concentration in the anolyte is exceedinglyflexible and easily carried out, and by employing it a very accurate control of the process may be achieved. If the dissolved lead concentration should decrease unduly, as might occur if an excessive quantity of catholyte were added to the anolyte, it may be allowed to increase slowly by electrolysis, or it may be in creased quickly by adding an appropriate amount of acetic acid to the anolyte in the cell.

It is desirable,though not essential, to avoid' having to add verylarge quantities of catholyte to anolyte, for each addition of catholyte increases the bulk of the anolyte and decreases the concentration of sodium .acetate therein. This difficulty is notserious, however, because the volume of the anolyte may easily be maintained by permitting evaporation of water therefrom. Furthermore, minor variations in the concentration of sodium acetate in the anolyte do not materially affect the operation of the process.

The concentration of dissolved electrolyte in both anolyte andcatholyte is dictated primarily and to make up the deficiency of hydrostatic head ofthe I the recovery thereof.

. tercurrent principle.

by economic considerationsbalancing ohmic resistance of the electrolyte against losses of electrolyte through spillage. If the concentration of the electrolyte is low, loss through spillage will be low, but the ohmic resistance, and hence the power consumption, will be relatively high. Conversely, a high concentration of electrolyte Will result in decreased ohmic resistance and de-.

Temperature of electrolytes C 40 Current density per sq. ft amperes-- 50 Anolyte composition: J`

Sodium acetate per cent-- 10 Alkalinity (as NaOH) per cent 0. 1

Catholyte composition, sodium hydroxide per cent 5 Care should be taken in carrying out the process to avoid the formation of sponge lead at the cathode. This may be accomplished by taking care that anolyte containing dissolved lead does not come in contact with any part of the cel/l which might act as a cathode. Maintaining a hydrostatic head in the catholyte, thus insuring that seepage through the diaphragm proceeds from catholyte to anolyte and not from anolyte to catholyte, is of. value for this reason in addition to the reasons set forth above. It is also Aadvisable to insulate the diaphragm from direct contact with the cathode, and to provide bolts, rods, studs, or other metallic parts which might come in contact with the anolyte with hard rubber or other insulating coverings. To the same end it is advantageous to employ anodes of slightly smaller diameter than the cathodes, thereby to insure that the current will not have to pass around any`of the frame parts of the diaphragm, if, as is preferred, the diaphragm is in the'form of an envelope surrounding the cathode.

Treatment of the solid product of the cell does not entail particular difficulty. It settles readily from the anolyte, and any standard design of settling tank or thickener may be employed ln Conical settling tanks have been found to be satisfactory.

The product of the settling tanks filters easily, and any of the Vcommonly available filters may be employed to separate the solid and liquid components thereof; for example, a continuous vacuum lter may be employed successfully. None of the salts present in the anolyte is adsorbed by the solid flltered product, and consequently it may be washed easily and effectively, employing ordinary tap Water for th'e purpose and preferably carrying out the washing step on the coun- Substantially all of ,the sodium acetate may be removed from the solid product by washing and may be returned to the anolyte for reuse. Ordinarily, no precautions need be taken to guard against the presence of carbonate in the wash water, as the quantity of such impurity present in ordinary tap Water is so small that it Will not cause serious contamination of the litharge. In some localities, however, such precautions may be made necessary by theI character of the water available.

Drying of the filtered product may be accomplished in any of the commercially obtainable driers. 'Ihe only precaution necessary in this operation is to avoid the use of temperatures very 'are present are converted to litharge.4 If the product is heated strongly during the drying thereof, the lead hydrate will be converted to lead peroxide which will contaminate the litharge.

The dried product is normally in a fine state of subdivision, but if it is desired to reduce the size of the particles still further, this may be accomplished by means of suitable grinding equipment, for example a ball mill or a pebble mill. Color changes in the litharge may also be brought about by grinding; for example, yellow litharge canY be changed to various shades of buil' or salmon.

The process of the invention affords a number of advantages not heretofore attained. The only materials consumed in the processare the lead of the anodes and water. 'I'he other reagents are employed cyclically and need not be renewed or replaced except in such quantities as may be lost through spillage. It is not necessary vto employ lead of particularly high quality for the anodes, because such impurities as it may contain accumulate as an anode slime which may be removed from the electrolytic cell without contamination of the cell product'. It may in some cases be profitable to treat these slimes for the recovery of antimony, bismuth, or other valuable substances which they may contain.

'I'he process proceeds smoothly and may be operated either continuously or intermittently as desired. The rate at which litharge is produced in a given cell may be controlled Within limits by regulating the current density and the rate of transfer of sod1um hydroxide from anolyte to catholyte, thus permitting some correlation of cell output with the demand for the product at any particular time. The electrolytic eiiiciency of the process is high and the general operating and labor costs are low; consequently, the process isextremely economical in operation.

The product of the cell is. of uniformly high quality. Although yellow crystalline litharge is the preferred cell product in accordance with the present invention, other lead compounds, such, for example, as White lead hydroxide or red litharge, may also be produced by suitably -adjusting the concentration of dissolved lead in the anolyte. Physically, the product obtained in accordance with the present invention is very nely divided and of remarkably uniform particle size, well suited to the uses to which litharge ordinarily is put. Chemically the product is extremely pure, being of. substantially theoretical chemical composition.A It possesses a high degree of solubility in acids, dissolving about ten times faster than ordinary commercial litharge. 'Ihis property is of considerable value in the preparation of lead salts such as lead nitrate, lead chromate, and the like from litharge. The yellow crystalline litharge produced in accordance with the invention possesses the property of darkening upon `exposure to light. The fine yellow crystals of litharge may be converted to the buff color of commercial litharge by grinding, and the shade of color of the nal product may be controlled by proper adjustment of the grinding conditions. lConsequently, it is' possible consistently to produce litharge of uniform color characteristics.

We claim:

1. The method of producing litharge which comprises passing an electric current through a cell comprising a lead anode, an anolyte comprising a substance capable of serving upon electrolysis as a solvent for lead, a diaphragm, a catholyte comprising a reagent capable of p recipitating litharge, and an insoluble cathode, and controlling the progress of reactions taking place within the cell during electrolysis by maintaining the concentrationof dissolved lead in the anolyte between about 0.1% and 1.0%.

2. The method of producing litharge which comprises passing an electric current through a cell comprising a lead anode, an anolyte comprising a substance capable of serving upon' electrolysis as a solvent for lead, a diaphragm, a catholyte comprising a reagent capable of precipitating litharge, and aninsoluble cathode, and controlling the progress of reactions taking place Within the cell during electrolysis by maintaining the concentration of dissolved lead in the anolyte between 0.2% and 0.4%.

3. Themethod of producing litharge which comprises passing an electric current through a cell comprising a lead anode, an anolyte comprising a substance capable of serving lupon electrolysis as a solvent for leada diaphragm, a catholyte comprising a reagent capable of precipitating litharge, and an insoluble cathode, and

controlling the progress of reactions taking place within the cell during electrolysis by maintaining the concentration of dissolved lead in the anolyte at about 0.3%.

4. In an electrolytic process for the production of litharge in a bifluid electrolytic cell comprising an anolyte containing a substance capable upon electrolysis of serving as a solvent for lead and a catholyte containing an alkali metal hydroxide, the improvement which comprises maintaining the concentration of dissolved lead in the anolyte within predetermined limits by maintaining a hydrostatic head of catholyte with respect to anolyte in the cell `to insure the passage of hydroxyl ions from the catholyte to the anolyte at a rate approximating rate at which lead dissolved in the anolyte is ca pable of combining therewith to produce litharge.

5. In an electrolytic process for the production of litharge in a biiiuid electrolytic cell comprising an anolyte containing a substance capable upon electrolysis ofserving as a solvent for lead and a catholyte containing an alkali metal hydroxide, the improvement whichcomprises maintaining the concentration of dissolved lead in the anolyte within predetermined limits .by maintaining a hydrostatic head of catholyte with respect to anolyte in the cell to insure the passage of hydroxyl ions from the catholyte to the anolyte at a rate approximating but not exceeding a rate sumcient to maintain the concentration of dissolved lead within the anolyte between about 0.1% and 1.0%.

6. In an electrolytic process for the production of litharge in a bifluid electrolytic cell comprising an anolyte containing a substance capable upon `electrolysis of serving as a solvent for lead and a catholyte containingV an alkali metal hydroxide, the improvement which comprises maintaining the concentration of dissolved lead in the anolyte within predetermined limits by maintaining a hydrostatic head of catholyte with respect to anolyte in the cell to insure the passage but not exceeding the of hydroxyl ions from the catholyte to `the anolyte at a rate approximating but not exceeding a 'rate sufiicient to maintain .the concentration of dissolved lead within the anolyte between about 0.2% and 0.4%.

` 7. In an electrolytic process for the production of litharge in a biluid electrolytic cell comprising an anolyte containing a substance capable upon electrolysis of serving as a solvent for lead anda catholyte containing an alkali metal hydroxide, the improvement which comprises maintaining the concentration of dissolved lead within predetermined limits by adding a portion of the4 catholyte which contains hydroxyl ions to anolyte at a point external to the reacting zones of the' cell,'said catholyte being added to said anolyte in an amount sufcient to maintain the concentration of dissolved lead in the anolyte between about 0.1% and 1.0%.

8. In an electrolytic process for the production of litharge in a biiiuid electrolytic cell com-- -to said anolyte in an amount sufficient to maintain the concentration of dissolved lead in the anolyte between about .0.2% and 0.4%.

9. In an electrolytic process for the production of litharge in a bifluid electrolytic cell comprising an anolyte containing a substance capable upon electrolysis of serving as a solvent for lead and a catholyte containing anI alkali metal hydroxide, the improvement which comprises maintaining the concentration of dissolved lead within predetermined limits by adding catholyte containing'hydroxyl ions to anolyte at a point external to the `reacting zones of the cell, said'catholyte being added to said anolyte in an amount suicient to maintain the concentration of dissolved lead in the anolyte at about 0.3%.

10; In an electrolytic process for the production of litharge in a bifluid electrolytic cell containing anolyte comprising a substance capable lof serving upon electrolysis as a solvent for lead in contact with an anode, catholyte comprising an alkali metal hydroxide fon contact with a cathode, and a diaphragm separating the anolyte and catholyte, the improvement which comprises maintaining a hydrostatic head of the catholyte with respect to the anolyte in the cell to insure the passage of hydroxyl ions from the catholyte to the anolyte at a rate approximating but not exceeding the rate at which lead dissolved in the anolyte is capable of combining therewith to produce litharge, and adding catholyte to the anolyte at a point external to the reacting zones of the cell in amount suicient'to maintain the dissolved lead content of the anolyte within predetermined limits. l

11. In an electrolytic process for the production of litharge in a biuid electrolytic cell containing anolyte comprising a substance capable upon electrolysis of serving as a solvent for lead in contact with a lead anode, catholyte comprising an alkali metal hydroxide in contact with a to insure the passage of hydroxyl ions from catholyte to the anolyte at a rate approximating but not exceeding the rate at which lead dissolved in the anolyte is capable of combining therewith to produce litharge, olyte to the anolyte at a point external to the reacting zon'es of the cell in amount sumcient to maintain the dissolved lead content of the anolyte between about 0.1% and 1.0%.

12. In an electrolytic process for the production of litharge in a biuid electrolytic cell containing anolyte conprising sodium acetate in contact with a lead anode, catholyte comprising sodium hydroxide in contact with a cathode. and a diaphragm separating the anolyte and catholyte, the improvement which comprises maintaining a hydrostatic head of the catholyte with respect to the anolyte in the cell to insure the passage of hydroxyl ions from the catholyte to and adding cath.

the l the anolyte at a rate approximating but not exceeding the rate at which lead dissolved in the anolyte is capable of combining therewith to produce litharge, and adding catholyte to the anolyte at a point external to the reacting zones of the cell in amount suillcient to maintain the dissolved lead content of the anolyte between about 0.2% and 0.4%.

13. The method of producing litharge electrolytically which comprises introducing an anolyte containing about 10% sodium acetate into the anolyte compartment of a biiluid electrolytic cell, introducing a catholyte containing about 5% sodium hydroxide into the catholyte compartment of said biuid electrolyticalcell, and establishing a concentration of dissolved lead in the anolyte between 0.2% and 0.4%.

REGINALD G. BOWMAN. WILLIAM J. KNOX,- Jn. 

